This invention relates to a device for detecting the concentration of oxygen in a combustion gas, which device is of the type having at least one layer of an oxygen ion conductive solid electrolyte and is particularly suitable for use in an exhaust system of a combustion engine to detect an actual air/fuel ratio of an air-fuel mixture supplied to the engine by detecting the concentration of oxygen in the exhaust gas.
In the field of internal combustion engines, and particularly in automotive engines, it has been popularized to detect changes in the air/fuel ratio of an air-fuel mixture actually supplied to the engine as the basis for feedback control of the air/fuel ratio by detecting changes in the concentration of oxygen in the exhaust gas. This is because usually it is more convenient to provide an oxygen sensor to the exhaust system of the engine than to the intake system. Most of the oxygen sensors conventionally used for this purpose are of the concentration cell type having a layer of an oxygen ion conductive solid electrolyte, a measurement electrode layer porously formed on one side of the solid electrolyte layer and a reference layer formed on the other side. These oxygen sensors are designed and used such that the measurement electrode is exposed to the exhaust gas while the reference electrode is exposed to a reference gas having a known oxygen partial pressure, usually atmospheric air, whereby an electromotive force is generated across the two electrode layers because of a difference between the reference oxygen partial pressure in air and an equilibrium oxygen partial pressure in the exhaust gas. Since the magnitude of the equilibrium oxygen partial pressure in the exhaust gas depends on the air-fuel ratio of an air/fuel mixture from which the exhaust gas is produced, the electromotive force serves as an electrical signal representative of the air/fuel ratio. However, the potential of this electromotive force is not directly proportional to the air/fuel ratio. A great and sharp change occurs in the potential of the electromotive force on the occurrence of a change in the air/fuel ratio across a stoichiometric air/fuel ratio (where the excess air factor .lambda. of the air/fuel mixture becomes 1.0), that is, a change from a fuel-rich mixture to a lean mixture or in the reverse direction. Accordingly this type of oxygen sensors is suitable for use in engines operated with a stoichiometric or nearly stoichiometric air-fuel mixture. However, when the air-fuel ratio varies only on one side of the stoichiometric ratio the electromotive force generated by the sensor disposed in the exhaust gas exhibits only very little changes in its magnitude. Therefore, it is practically impossible to detect air/fuel ratios deviating from a stoichiometric air/fuel ratio by disposing an oxygen sensor of the above described type in the exhaust gas.
Meanwhile, in automotive gasoline engines the development of so-called lean-burn engines has been in progress mainly with the view of improving the thermal efficiency by the employment of a considerably lean air-fuel mixture (large in the value of the excess air factor .lambda.), and attention has been given also to the so-called rich-burn engines which are operated with a considerably rich air-fuel mixture (small in the value of .lambda.) and exhibit high mechanical efficiencies.